Recent developments in the field of video signal processing have produced digital high definition television signal processing and transmission systems. One such system is described in U.S. Pat. No. 5,168,356--Acampora et al. In that system, a codeword datastream, including codewords compatible with the known MPEG data compression standard, is conveyed to a transport processor. A major function of the transport processor is to pack variable length codeword data into packed data words. An accumulation of packed data words, called a data packet or data cell, is prefaced by a header containing information identifying the associated data words, among other information. Thus an output from the tranport processor is a packetized datastream comprising a sequence of transport packets. The transport packet format enhances the prospects of resynchronization and signal recovery at a receiver, e.g., after a signal disruption which may result from a transmission channel disturbance. This is accomplished by providing header data from which a receiver can determine re-entry points into the datastream upon a loss or corruption of transmitted data.
A high definition television (HDTV) terrestrial broadcast system recently proposed as the Grand Alliance HDTV system in the United States employs a vestigial sideband (VSB) digital transmission format for transmitting a packetized datastream with a prescribed data field structure. The Grand Alliance HDTV system is a proposed transmission standard that is under consideration in the United States by the Federal Communications Commission (FCC) through its Advisory Committee of Advanced Television Service (ACATS). A description of the Grand Alliance HDTV system as submitted to the ACATS Technical Subgroup Feb. 22, 1994 (draft document) is found in the 1994 Proceedings of the National Association of Broadcasters, 48th Annual Broadcast Engineering Conference Proceedings, Mar. 20-24, 1994.
In the Grand Alliance system, data is arranged as a sequence of data fields. Each field structure includes 313 segments: a field sync segment (which does not contain payload data) followed by 312 data segments. Each data segment includes a data component and a forward error correction (FEC) component. A synchronizing (sync) component prefaces each data segment. A transport processor provides fixed length 188 byte packets to a transmission processor, which performs various coding functions on each packet to produce output symbol segments to be conveyed to an output transmission channel. Each byte comprises a predetermined number of symbols, e.g., 4 symbols. The data packets contain data in conformance with the ISO-MPEG (International Standards Organization-Moving Pictures Experts Group) MPEG-2 data compression standard. The transport processor provides only the data packets to the transmission processor, which adds an overhead FEC error detection and correction component to each segment, and an overhead field sync segment at the beginning of each data field, ie., between each group of data field segments. The rate of data flow must be regulated to perform these operations since, as will be seen, the overhead FEC components and the overhead field sync segment occur at different times and exhibit different durations.
The packets are separated by an interval which permits overhead required by each data segment (e.g., FEC data) to be inserted into the datastream by the transmission processor. However, the packet datastream must be interrupted and delayed for a period of time equal to a segment interval when the longer duration field sync segment, which does not contain a data payload like the other packets, is to be inserted into the datastream. A datastream created by the need to interrupt and insert different duration overhead information (FEC and Field sync) at different times is illustrated by FIG. 3. This datastream comprises 312 packet byte plus FEC intervals between Field sync intervals, as will be discussed.